Practice question

Maintenance of biconcave disc shape maximizing surface to volume ratio for deformability and gas exchange depends on membrane skeleton elasticity and organization. Spectrin consists of alpha two hundred eighty kilodalton and beta two hundred forty six kilodalton subunits each containing twenty triple-helical spectrin repeats of one hundred six residues forming elongated flexible anti-parallel heterodimer one hundred nanometers long, two heterodimers associate head to head via helical bundle forming tetramer about two hundred nanometers acting as entropic spring with persistence length about ten nanometers. Tetramers interconnected at junctional complexes comprising thirteen subunit short F-actin filaments capped by adducin and tropomodulin plus protein 4.1R dematin tropomyosin forming pseudohexagonal lattice of about thirty five thousand nodes per cell beneath lipid bilayer. Linkage to bilayer via ankyrin-Band 3 and protein 4.1R-glycophorin C ensures force transmission. Hereditary mutations in SPTA1 encoding alpha spectrin and SPTB encoding beta spectrin cause hereditary spherocytosis and elliptocytosis with fragile cells. GPCR seven-pass signaling and aquaporin channel not structural, confirming spectrin role maintaining biconcave geometry and mechanical elasticity under shear stress circulation.